Fix typos

Summary:
The changes to developer-notes.md are not applicable to our version, because these paragraphs have been modified and don't contain the typos.

This is a backport of Core PR14395

Test Plan: Proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8432